Okieeomi
We've checked with the development team and there are few reasons why this is difficult to implement:
Under the current system, even if a "Take All" button were added, it would not change the processing speed when receiving all of the items.
Additionally, one would not be able to stop the process once the "Take All" is activated until all items have been placed in the player's inventory. This could be very problematic in situations involving large number of items. For example if there were 50+ items, it would take a long time to load and would prevent the player from moving while this is being processed.
For these reasons, unfortunately the development team is not considering implementing a "Take All" feature for the Delivery Box.